I. Field of the Invention
This invention relates to mud saver valves which allows the downward flow of drilling mud but closes when pumping is interrupted to retain the mud within the drill string and, in particular, to a mud saver valve with an inner sleeve which is replaceable when worn in order to ensure sealing engagement with the piston.
II. Description of the Prior Art
In the drilling of oil and gas wells, it is common practice to insert in the drill string between the kelly and the drill pipe a valve to retain mud in the kelly when the drill string is detached. The advantages of mud saver valves include saved mud costs, decreased chances of pollution, and increased safety to rig personnel. Many of the past known mud saver valves include a piston having an axial throughbore biased upwardly within the valve housing by a spring. When closed, the piston engages a closure plug which blocks mud flow through the piston. As the surface pumps pump drilling mud through the drill string, the fluid pressure acts against the top of the piston and the force of the spring to open the fluid passageway through the valve. When mud flow is interrupted, the spring forces the piston against the closure plug to close the passageway.
The closure plug is removably mounted within the upper end of the valve housing to permit the running of wireline tools through the drill string. Typically, the closure plug is provided with a spear head such that an overshot may be run to grasp the plug for removal. Many of the past known closure plugs also include a check valve to bleed off excess downhole fluid pressure when the valve is closed.
As the pumping pressure increases the piston is forced downwardly to permit mud flow past the closure member and through the piston passageway. Because of the abrasive properties of the drilling mud, the lower end of the closure plug can become eroded and worn resulting in an insufficient seal between the closure plug and the piston. The upper edge of the piston can also become worn due to the flow of the drilling mud. Past known mud saver valves have provided the upper end of the piston with a wear-resistant member which can be replaced. However, because of the construction of past known closure plugs, in the event the plug became worn causing leakage either the valve would have to be discarded or the entire closure plug would have to be replaced.